Generally, in an assembly process of a disk drive such as a hard disk drive (i.e., a magnetic disk drive, usually abbreviated as HDD), a disk such as a magnetic disk is mounted on the hub portion of a spindle, while a magnetic circuit and an actuator constituting a driving coil motor such as a voice coil motor are mounted on the base of a disk enclosure. The term “actuator” used here refers to a component element that includes a head support (head suspension) on which a magnetic head is held and a flat coil formed on the side of the head support opposite to the magnetic head. In some cases, the actuator may includes the magnetic circuit.
To describe in further detail, the magnetic circuit includes an upper magnet and upper magnetic yoke disposed above the actuator, a lower magnet and lower magnetic yoke disposed under the actuator, and a magnetic circuit connecting portion magnetically connecting between the upper magnetic yoke and the lower magnetic yoke. Generally, the upper magnetic yoke, the lower magnetic yoke, and the magnetic circuit connecting portion are fabricated from a magnetic metal such as iron to provide a path for the magnetic flux produced by the magnetic field arising from the upper and lower magnets. On the other hand, the base of the disk enclosure is usually fabricated from a nonmagnetic metal such as aluminum. In other words, the magnetic circuit and the base are respectively fabricated from dissimilar metals.
In a prior art disk drive assembly process, the magnetic circuit is fixed to the base by fastening the former (in particular, the lower magnetic yoke portion) to the latter by means of a plurality of screws. The fastening of such dissimilar metals is accomplished by forcefully fastening together the dissimilar metals by the screws. The thermal expansion coefficient of the base formed from aluminum (about 21×10−6° C.−1) is typically about twice that of the magnetic circuit formed from iron (about 12×10−6° C.−1). When the ambient temperature, etc., change, the base tries to expand about twice as much as the magnetic circuit; however since the base and the magnetic circuit are forcefully fastened together by the screws, the base can only expand by the same amount as the magnetic circuit.
As a result, the magnetic circuit compresses the base because of the difference in thermal expansion coefficient between the dissimilar metals, causing stress in the shearing direction at the fastening portion of the dissimilar metals. If this shearing stress becomes greater than the retaining force acting in the shearing direction at the screw-fastened portion, a microscopic sliding movement (also called stick-slip) occurs at the fastened portion. Since such a microscopic sliding movement occurs by involving impulsive vibration, the head position of the magnetic head fluctuates due to the impulsive vibration, and the on-track performance of the magnetic head degrades, resulting in the problem that the magnetic head cannot be positioned stably over the target track on the disk.
For reference purposes, Japanese Unexamined Patent Publication No. 2007-292221 (hereinafter called Patent Document 1) will be presented below as a document related to the prior art.
In Patent Document 1, there is disclosed a structure for use in a recording media drive having a first component (spindle hub) provided with a threaded hole, a second component (clamp) that contacts the first component, and a clamping screw that is screwed into the threaded hole of the first component to fix the second component to the first component, the structure being such that, when clamping the components together by screwing the clamping screw into the threaded hole of the spindle motor of the recording media drive, a lubricating film is formed between the threaded hole and the clamping screw so as to reduce the frictional force between the threaded hole and the clamping screw, thereby providing an increased axial force (the clamping force in the axial direction) without increasing the clamping torque of the clamping screw.
However, in Patent Document 1, no mention whatsoever is made of specific techniques for suppressing the occurrence of a microscopic sliding movement involving impulsive vibration at the fastening portion of the dissimilar metals due to the shearing stress occurring at the fastening portion because of the difference in thermal expansion coefficient between the first component (spindle hub) and the second component (clamp).
Accordingly, with the structure disclosed in Patent Document 1, it is not possible to address the problem that arises when a microscopic sliding movement involving impulsive vibration occurs because of the difference in thermal expansion coefficient between the magnetic circuit and the base when fixing the magnetic circuit to the base in accordance with the prior art.
It is therefore desired to provide a disk-drive magnetic circuit fixing structure and a magnetic circuit fixing method that can suppress the occurrence of a microscopic sliding movement involving impulsive vibration at the fastening portion between the magnetic circuit in the disk drive and the base of the disk enclosure, for example, when the thermal expansion coefficient of the magnetic circuit differs from that of the base.